C++17: Lambdas

C++ has supported lambda functions since C++11, and post-C++14 even supported generic lambdas. C++17 adds a quirky feature to enable ‘copy capture’ of ‘this’. Here’s an example of it in action:

struct MyStruct
{
    auto lambda_with_this() // Before C++17, copies this pointer
    {
        auto f = [this]{ return value; };
        return f;
    }
    
    auto lambda_with_star_this() // C++17 - takes a local copy of *this
    {
        auto f = [*this]{ return value; };
        return f;
    }
    
    int value;
};

TEST( Cpp17, lambda_capture_this )
{
    MyStruct s{ 42 };
    auto f = s.lambda_with_this();
    s.value = 101;
    EXPECT_EQ( 101, f() );
}

TEST( Cpp17, lambda_capture_star_this )
{
    MyStruct s{ 42 };
    auto f = s.lambda_with_star_this();
    s.value = 101;
    EXPECT_EQ( 42, f() );
}

Notice that in the second case, we capture a copy of our object – so the lambda returns the value held at the point of capture (42) rather than the value when it is called (101). This can be very important if ‘this’ has been destroyed between the creation of the lambda and the time at which it’s called.

Now, C++14 also supported generalised lambda capture, which meant you could (re-)name variables when capturing (and provided a neat way to capture-by-move):

    auto f = [tmp = *this]{ return tmp.value; };

But the C++17 code is more concise. See this useful StackOverflow post for more discussion.

Another advance in C++17 is that lambdas are implicitly constexpr – so you can now use them in compile-time contexts, like declaration of std::array:

// lambda explicitly constexpr since C++17
auto square = []( auto v ){ return v*v; }; 

TEST( Cpp17, lambda_implicitly_constexpr )
{
    // std::array calls 'square()' at compile time
    std::array<int, square(4)> values; 
    EXPECT_EQ( 16, values.size() );
}

See also the previous C++17 post.

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C++17: Nested Namespaces

Another crowd-pleaser in C++17 is the ability to declared nested namespaces without literally nesting them. In the past, you had to do this, which involves a lot of wasted whitespace:

namespace NestedNamespaces
{
    namespace Really
    {
        namespace Work
        {
            auto return_int(){ return 42; };
        }
    }
}

Happily, you can now do this instead:

namespace NestedNamespaces::Really::Work
{
    auto return_int(){ return 42; };
}

TEST( Cpp17, nested_namespaces )
{
    EXPECT_EQ( 42, NestedNamespaces::Really::Work::return_int() );
}

See also previous C++17 post.

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C++17: Structured Bindings

This is one of a series of posts on C++17 features – see also previous post on if initialisers.

Structured bindings are a convenient way of handling multiple return values from functions. Whilst F# has been able to do this:

let f() = 42, "Hello, World" // return a pair of values
let a, b = f() // assign a and b to the values returned by f

in C++, we’ve had to declare the variables up front and use std::tie to assign values (so not only does this take more lines, we also have to default initialise the variables then throw away the defaults).

auto t = std::make_tuple( 42, "Hello, World" );
int a, b;
std::tie( a, b ) = t;

The new structured bindings are much more concise, even if the use of square brackets came as a surprise. Even better, you can use structured bindings with structs and std::array.

int my_int{ 42 };
std::string my_string{ "Hello, World" };
bool my_bool{ true };

auto return_pair()
{
    return std::make_pair( my_int, my_string );
}

auto return_tuple()
{
    return std::make_tuple( my_int, my_string, my_bool );
}

struct MyStruct
{
    int a;
    double b;
    int c;
    
    static MyStruct Expected;
};

MyStruct MyStruct::Expected = { 1, 2.2, 3 };

auto return_struct()
{
    return MyStruct::Expected;
}

auto return_array()
{
    return std::array<int,3>{ 1, 2, 3 };
}

auto return_map()
{
    return std::map<int, std::string>{ {1, "a"}, {2, "b"}, {3, "c"} };
}

TEST( Cpp17, structured_bindings_for_pair )
{
    auto [i, s] = return_pair();
    
    EXPECT_EQ( my_int, i );
    EXPECT_EQ( my_string, s );
}

TEST( Cpp17, structured_bindings_for_tuple )
{
    auto [i, s, b] = return_tuple();
    
    EXPECT_EQ( my_int, i );
    EXPECT_EQ( my_string, s );
    EXPECT_EQ( my_bool, b );
}

TEST( Cpp17, structured_bindings_for_struct )
{
    auto [i1, d, i2] = return_struct();
    
    EXPECT_EQ( MyStruct::Expected.a, i1 );
    EXPECT_EQ( MyStruct::Expected.b, d );
    EXPECT_EQ( MyStruct::Expected.c, i2 );
}

TEST( Cpp17, structured_bindings_for_array )
{
    auto [i1, i2, i3] = return_array();
    
    EXPECT_EQ( 1, i1 );
    EXPECT_EQ( 2, i2 );
    EXPECT_EQ( 3, i3 );
}

TEST( Cpp17, structured_bindings_for_iterating_over_map )
{
    for ( const auto& [key,value] : return_map() )
    {
        switch (key)
        {
            case 1: EXPECT_EQ( "a", value ); break;
            case 2: EXPECT_EQ( "b", value ); break;
            case 3: EXPECT_EQ( "c", value ); break;
            default: break;            
        };
    }
}

For me, the best examples come when combining features – the range-based for loop with structured bindings is a thing of beauty.

See also next C++17 post.

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C++17: if initialiser

I attended a C++17 presentation by Nicolai Josuttis last year, but at the time, my laptop’s compiler didn’t support any of the features to try them out. After a recent update, it turns out that many are now supported, so I’ve written a few unit tests using GTest.

The first feature I tried was the if initialiser. This feature looks a bit odd at first, because C++ programmers are so conditioned to seeing if statements containing a single condition. Allowing an initialiser statement as well

if ( initialiser; condition )

means that you can initialise a variable and test it on the same line. It also prevents the variable being used outside the scope of the if statement – this prevents accidental re-use if you subsequently mis-type a variable name.

auto return_int()
{
   return 101;
}

TEST( Cpp17, if_initialiser )
{
    // NB we can use i in the body of the if or the else
    // Also, must have a variable name for the object to live in the whole statement
    // (so must name locks taken, even if not used in the body, otherwise it's a temporary).
    if ( auto i = return_int(); i < 100 )
    {
        EXPECT_TRUE( i < 100 );
    }
    else
    {
        EXPECT_TRUE( i >= 100 );
    }
}

TEST( Cpp17, if_initialiser_with_map_insert)
{
    std::map<int, std::string> my_map{ {42, "Hi" } };
    
    if ( auto[it, inserted] = my_map.insert( std::make_pair(42, "Bye" ) ); !inserted )
    {
        // See also StructuredBindings for iterating over a map
        auto& [key,value] = *it; // iterator is pair of key and value
        EXPECT_EQ( 42, key );
        EXPECT_EQ( "Hi", value );
        
        value = "Bye"; // update the value
        EXPECT_EQ( "Bye", my_map[42] );
        
        // key = 43; // compile error! - cannot assign to const key-type
    }
}

See also next post on C++17 Structured Bindings.

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Book Review: The Lost Fleet – Valiant, Jack Campbell

This book follows the same template as the other Lost Fleet books that I’ve read. We travel with Captain “Black Jack” Geary on Dauntless, the flag ship of his fleet, as he attempts to steer his people home after a damaging series of battles. This episode is more optimistic, the fleet is performing well in battle and Geary’s efforts to bring a humane change in culture to the personnel is succeeding (they rescue stranded enemy civilians who had been abandoned on an outpost planet). However, he still faces the growing threat of hidden enemies within the leadership who are still trying to overthrow him. Geary confronts the fact that he cannot continue to court Co-President Victoria Rione now that his growing feelings for Captain Tanya Desjani are obvious to everyone in his crew.
Three stars

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Book Review: The Fix, David Baldacci

TheFixI really enjoyed the first in David Baldacci’s Amos Decker series, so was looking forward to this one. Unfortunately, it wasn’t at the same level – a completely implausible plot, combining industrial espionage with an international spy ring, terrorism and family betrayal.
TwoStars

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IET Turing Lecture: Innovation and Technology – Art or Science

This evening at the IET was split into two lectures, the first by Larissa Suzuki entitled “Brave New Smart World”. Larissa covered a lot of ground, scaring the audience with figures on our over-population of the planet, over-consumption and lack of recycling. Then she showed how we may have potential to engineer our way out of these global problems – with smart cities, self-driving cars and smart-recycling.
Quote of the Day:

You cannot jujitsu your way out of bad data


The main Turing Lecture was by Andy Harter, who among other things invented VNC (used for connecting to remote computing devices). The stated intention of the lecture was to make the audience think about whether innovation and technology owe more to Art or Science. The lecturer admitted that his hope was that we’d agree that both are necessary. He covered a number of facets that he believes are the ingredients behind the best inventions: Creativity, Necessity, Storytelling, Timing, Retrospection, Observation, Laziness, Simplicity, Responsiveness and Generosity.

If necessity is the mother of all inventions, laziness is the father!

This quote was quite pertinent to the speaker, who invented VNC so that he didn’t have to leave his office to help a colleague down the corridor with issues on his machine!

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